Navigation systems, autonomous driving and safety applications in vehicular and other mobile environments require high-accuracy positioning. The commonly used global navigation satellite system (GNSS) positioning is inherently limited when the sky is obstructed, for example in deep urban canyons or in underground parking lots. Compensation for lack of GNSS availability or degraded performance to maintain positioning may be performed with a Dead-Reckoning (“DR”) algorithm using sensors, but the positioning accuracy is severely degraded.
Known WLAN usage does not include high-accuracy ranging. Vehicular safety applications goals for positioning (distance and location) include a goal of lane level accuracy, i.e. sub-meter error. This goal translates into a 3.2 nanosecond (ns) maximal time measurement error, which is challenging when considering that WLAN is a low-cost technology without high accuracy clock synchronization.
Mechanisms for time-of-flight (ToF) measurement are known. For example, “Wireless Positioning Technologies and Applications” by Alan Bensky. Artech House, 2007, (hereinafter “Bensky”) describes a method that uses unformatted orthogonal frequency divisional multiplexed (OFDM) symbols to measure distance between a reference point and a mobile unit (e.g. a vehicle). Other publications such as “Measuring Round Trip Times to Determine the Distance between WLAN Nodes” by Andre Gunther and Christian Hoene in NETWORKING 2005: Networking Technologies, Services, and Protocols; Performance of Computer and Communication Networks; Mobile and Wireless Communications Systems pp 768-779, (hereinafter “GH”) describe round-trip-time (RTT) measurements based on sampling of packet transmit and receive events.
The models described by Bensky do not consider the complex mitigation of multipath errors. Gunther and Hoene attempt to develop schemes to minimize the impact of such errors but the inherent error is still far higher than required. Thus, known TOF-based methods for obtaining accurate positioning suffer from significant disadvantages.
ToF is measured by a time measurement unit. The principle of operation of a time measurement unit is described by multiple articles, including by GH, which suggests 1 μs accuracy, achievable with common WLAN modems. As mentioned, the maximal required error of 3.2 ns renders this 1 μs accuracy irrelevant. A single clock cycle is used for measurement. The clock cycle duration is a function of the channel bandwidth. For a 10 MHz channel as commonly utilized in IEEE 802.11p, the clock cycle is 100 ns, which translates to 30 meters accuracy. Accordingly, there is a need to provide a high-quality positioning solution (in particular for IEEE 802.11p) and therefore a time measurement unit alone, without supporting mechanisms, does not meet the accuracy goal. Known WLAN modems have to be modified (enhanced) to enable the required high accuracy positioning measurement.